Envelope bessel xwave

Description

This MR adds an envelope to Bessel source in the code. Moreover, it refactors the envelope calculations so that plane wave carrier and envelope are calculated in separate routines.

Bessel envelope looks like this, when the same envelope is applied to Bessel beam and plane wave:

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.